A. General
Reference is frequently made herein to circuit breakers, sectionalizers, and reclosers. All of these devices are designed to switch distribution circuits on and off by opening or closing switches therein. Typically modern breakers, sectionalizers, and reclosers do not contain any means to determine when or if they should open or close. Instead these devices are attached to control devices which measure power system currents and/or voltages, and send signals to the reclosers, sectionalizers, and circuit breakers to open and close. The methods described below, which decide when to open and close the circuit breakers, sectionalizers, and reclosers, are typically implemented in these control devices. The usual practice in the art is to refer to a device that controls a circuit breaker as a protective relay. Similarly, a device that controls a recloser is typically referred to as a recloser control and a device that controls a sectionalizer is typically referred to as a sectionalizer control.
Thus, even though reclosers, sectionalizers, and breakers are frequently referred to herein as measuring current, sensing short circuit current, measuring voltage, and sensing when a line is energized or deenergized, it will be readily apparent to those skilled in the art that the control device associated with each circuit breaker, recloser, or sectionalizer is actually performing the measurements, detecting short circuits or line energization states, and making the decision to open and close the connected recloser, sectionalizer, or circuit breaker. Thus, when a circuit breaker, recloser, and/or sectionalizer is referred to herein, it means the combination of recloser and recloser control, sectionalizer and sectionalizer control, and circuit breaker and protective relay.
B. Power Distribution Systems with Radial Distribution Lines
Electric power distribution typically occurs at voltages in the range 4 kV to 35 kV. Historically, distribution lines were connected radially from distribution substations to loads. The prior art example of FIG. 1 illustrates a system 21 with a single distribution line 20 feeding several loads, Load 1 through Load 4, from a distribution substation 22. Usually several distribution lines radiate from the substation 22, but for simplicity, only one distribution line 20 is shown in FIG. 1.
A short circuit on such a radial line typically causes a power outage for all connected loads in a radial distribution system. Inside the substation 22 is a circuit breaker 24 that helps protect the distribution line 20 from short circuits. If a short circuit occurs anywhere on the line, then large currents will begin to flow from the substation 22 to the short circuit on the distribution line 20. Over-current detecting equipment, called protective relays, will detect the large current and will signal the circuit breaker 24 to open. When the circuit breaker 24 opens, the distribution line 20 is disconnected from the power source within the substation 22. This interrupts power to all of the loads, Load 1 through Load 4, connected to the radial distribution line 20. The loads then remain without power until a line crew travels to the site of the short circuit, repairs the short-circuited conductors, and then closes the circuit breaker 24. The duration of such a power outage is typically several hours.
Power can be restored to the loads faster after temporary short circuits by reclosing the circuit breaker. Still referring to FIG. 1, many short circuits are transient in nature. If the short-circuited line is disconnected from the power source by the circuit breaker 24, temporary short circuits can be self-healed. When the circuit breaker 24 recloses, the short circuit will be gone, and power will be restored to the loads immediately without the need to dispatch a line crew to repair the short circuit. This returns power to the loads faster, in a matter of seconds instead of hours.
Of course, there is no guarantee that the short circuit is temporary, and it may still be present when the circuit breaker closes. In that case, the circuit breaker 24 is again opened, and may be reclosed one or more additional times testing to see if the short circuit has self-healed. If the short circuit goes away, then the breaker remains closed. If the short circuit is permanent, then the breaker 24 opens a predetermined number of times, waiting a predetermined time between each closing, and then opens a final time and remains open. Again all of the connected loads are without power until a line crew locates and repairs the short circuit, and the circuit breaker is closed. This can take several hours. Since some portion of faults are temporary, reclosing of the circuit breaker 24 decreases the chances that a load will be without power for several hours, and increases the chance that the load will be without power for only a few seconds.
Sectionalizers and reclosers can be used to limit the size of the power outages caused by permanent short circuits. Some distribution lines are constructed with switches along the length of the line, as shown in FIG. 2. In particular, in FIG. 2, a system 23 with a sectionalizer switch 26 allows power to be restored to some loads faster. First, the operation of sectionalizer switches will be discussed, and then the operation of line reclosers will be discussed. Both reclosers and sectionalizers historically have the ability to measure the current flowing through the switch, and the voltage at least on one side of the switch. When the voltage is measured on only one side of the switch (the usual case), it is measured on the side of the switch closer to the sub-station 22 or to the power source.
The switch 26, also marked S, in FIG. 2 is a sectionalizer switch. It measures current flowing through the switch, and the voltage on the left (substation or power source) side of the switch. If a short circuit occurs between the substation 22 and the sectionalizer switch 26, the system of FIG. 2 behaves the same as the system of FIG. 1. If a short circuit 28 occurs between the sectionalizer switch 26 and the end of the distribution line 20 then the system of FIG. 2 acts differently than the system of FIG. 1. This short circuit 28 is said to be “down stream” of the sectionalizer 26.
The short circuit causes a large current to flow from the power source in the substation 22, through the substation circuit breaker 24, along the distribution line 20, through the sectionalizer switch 26, and to the short circuit 28. The sectionalizer 26 senses the large current flow, and then “knows” that the short circuit 28 is down stream from the sectionalizer location. If the short circuit location were upstream of the sectionalizer location, then the sectionalizer switch 26 would not have detected the large current.
As with the system 21 of FIG. 1, the circuit breaker 24 in the substation 22 opens and removes power from the distribution line 20, all of the connected loads, Load 1 through Load 4, and from the short circuit 28. This causes the voltage along the distribution line 20 to drop from several thousand volts to substantially zero volts. The sectionalizer 26 senses the absence of voltage, and “knows” that the substation breaker 24 has opened. The substation breaker 24 may be programmed to test several times for a temporary short circuit before finally opening and staying open if the short circuit 28 is permanent. The sectionalizer switch 26 counts each time the substation breaker 24 closes and opens. The sectionalizer 26 does this by sensing the short circuit current flowing through the sectionalizer switch when the substation breaker 24 closes, and senses the absence of voltage when the substation breaker 24 opens.
The sectionalizer switch S is programmed to open during one of the “open-intervals” of the substation breaker 24. This open-interval is the time when the substation breaker 24, or any device, is open. During some pre-determined open interval, the sectionalizer opens.
When the substation breaker 24 recloses with the sectionalizer switch open, there will be no large current flow because the short circuit 28 has been isolated from the power source by the open condition of the sectionalizer switch 26. The circuit breaker 24 will then remain closed. In other words, the sectionalizer switch 26 made the permanent short circuit 28 shown in FIG. 2 appear to be a temporary short circuit. The result is that after the substation breaker 24 closes, power is restored to Load 1 and Load 2 after just a few seconds, even though the short circuit 28 is permanent. Only the customers at Load 3 and at Load 4 will experience an extended power outage while the line crew searches for and repairs the short circuit 28. After the short circuit is repaired, the sectionalizer switch 26 is closed, and power returns to Load 3 and Load 4. This is an improvement over the system of FIG. 1 where a permanent short circuit caused an extended power outage for all of the loads.
FIG. 3 illustrates a system 27 with a recloser 30, also marked R, in place of the sectionalizer 26 in the system 23 of FIG. 2. This line recloser 30 further reduces the size and length of a power outage. For short circuits between the substation circuit breaker 24 and the recloser 30, the system 27 of FIG. 3 operates the same as the system 21 of FIG. 1. For short circuits downstream of the recloser 30, the systems operate differently. The short circuit 28 shown in FIG. 3 will cause a large current to flow from the substation 22 through the recloser 30 and to the short circuit 28. The recloser 30 senses the large current and opens quickly. The substation circuit breaker 24 is programmed to open with a longer time delay than the recloser 30, so the recloser opens and the substation breaker remains closed. This removes power from loads Load 3 and Load 4, but power to Load 1 and Load 2 is only degraded while the large current is flowing, and is completely restored when the recloser 30 opens. Typically the recloser will open in less than one second, so the short circuit 28 in FIG. 3 will only cause a power degradation lasting less than one second for Load 1 and for Load 2. The recloser 30 may be programmed to reclose after some time to test if the short circuit 28 is temporary. If the short circuit is temporary, then Load 3 and Load 4 will be without power for only a few seconds.
If the short circuit is permanent, then the recloser 30 will open and close a predetermined number of times, then open one final time and remain opened. Load 3 and Load 4 will remain un-powered for several hours while the line crew searches for and repairs the short circuit. Load 1 and Load 2 will experience a power degradation lasting less than one second each time the recloser 30 closes to test if the short circuit 28 still exists. This is an improvement over the system 23 of FIG. 2 where Load 1 and Load 2 experienced a power outage lasting several seconds.
C. Power Distribution Systems with Looped Distribution Lines
A system 31 in FIG. 4 shows a further improvement over the radial distribution systems of FIGS. 1-3 with looped distribution circuits providing rapid power restoration following permanent short circuits. The previously considered systems 21, 23 and 27 in FIGS. 1-3 involved radial distribution lines. The system 31 of FIG. 4 involves a looped distribution line 32. The term “looped” means that power can be fed to any of the loads, Load 1 through Load 6, from either direction. This system 31 is normally operated in the state shown in FIG. 4. In the normal operational state of the system shown in FIG. 4, reclosers R1, R2, R4 and R5 are closed, and recloser R3 is open. Recloser R3 is normally open because if all of the reclosers were closed, it would be difficult to control the amount of power flowing through the looped line, especially if the two circuit breakers 24 and 25 are in different substations.
Reclosers R1, R2, R4 and R5 can sense current flowing through them, and can also measure or at least sense the presence or absence of voltage on at least one side; typically the side closer to the substation 22 or the power source. Recloser R3 can measure current and can measure voltage on both sides of itself. Methods of measuring current and measuring or sensing voltage are, of course, well known in the art.
Recall that the system 27 shown in FIG. 3 performed very well for the short circuit 28 shown in FIG. 3. Load 1 and Load 2 experienced power degradation lasting less than one second even for a permanent fault. However, if the permanent short circuit 28 were upstream of the recloser 30 in FIG. 3, then all four loads would experience an extended power outage. So, the performance of system 27 depends on the location of the short circuit 28. The system 31 shown in FIG. 4 removes the dependence on short circuit location. This system performs substantially equally well regardless of where the short circuit exists or occurs.
For example, FIG. 5 shows a permanent short circuit 28 upstream of recloser R1. If the system 31 were not looped, then this short circuit would result in an extended power outage for loads Load 1, Load 2, and Load 3, because the substation breaker 24 would open and would de-energize the line serving all of those loads.
FIG. 6 and FIG. 7 show how the system 31 limits the size and duration of the outage for the short circuit 28 shown in FIG. 5. The short circuit 28 causes a large current to flow from the power source inside the substation 22 to the short circuit 28. The substation circuit breaker 24 opens for a pre-determined time, causing a temporary power outage for loads Load 1, Load 2 and Load 3. The substation breaker 24 typically tests the line 32 several times to see if the short circuit 28 is temporary. If the short circuit is temporary, then when the circuit breaker 24 closes the short circuit 28 will no longer exist, and the system 31 will return to the state shown in FIG. 4. If the short circuit 28 is permanent, the substation breaker 24 opens and closes a predetermined number of times, i.e., the breaker tests the line for a temporary short circuit, and then breaker 24 remains open. Reclosers R1 and R2 sense that the line connected to them is de-energized for an extended time. Reclosers R1 and R2 are both programmed to take specific action when they sense that the line 32 is de-energized for an extended time (e.g. longer than some predetermined time). Recloser R1 is programmed to open following a predetermined time delay after the line becomes de-energized. Recloser R2 is programmed to reconfigure to protect the section of line 32 between reclosers R2 and R1. Recloser R2 was previously configured to protect the section of line between reclosers R2 and R3 because the power source or substation 22 was to the left of recloser R2. Recloser R3 is programmed to close after a predetermined time when it senses that the line 32 connecting to either side of recloser R3 is de-energized. After the substation breaker 24 has finished testing the line for a temporary short circuit and breaker 24 is open, and all of the reclosers perform their programmed tasks, the system 31 is as shown in FIG. 7.
Notice that only Load 1 connected between recloser R1 and the substation breaker 24 experiences a prolonged power outage. Load 2 and Load 3 would have also experienced a prolonged power outage if the system were radial. However since the system 31 is looped, Load 2 and Load 3 are without power only for a few tens of seconds while circuit breaker 24 is open and while the predetermined time delays elapse before recloser R1 opens, recloser R2 reconfigures, and recloser R3 closes. A timeline for the entire process is shown in the bottom portion of FIG. 7.
Now assume a permanent short circuit 28 occurs between reclosers R1 and R2, as shown in FIG. 8. The permanent short circuit causes a large current to flow from the substation power source through the circuit breaker 24 and through recloser R1 to the short circuit 28. Recloser R1 is programmed to open with less delay than the substation breaker 24, so recloser R1 opens and circuit breaker 24 remains closed. I.e., when recloser R1 opens, the short circuit current ceases, so the substation breaker 24 does not open. Recloser R1 closes several times to test the line 32 for a temporary short circuit. In this example, the short circuit 28 is permanent, so recloser R1 eventually opens permanently. It opens before the substation breaker 24 is programmed to open, so the substation breaker remains closed.
The system 31 is now in the state shown in FIG. 9. Load 2 and Load 3 are de-energized. Recloser R2 performs exactly as in the previous example. If it senses that the line connected to it is de-energized for an extended time (e.g. longer than some predetermined time), it reconfigures to protect the section of line 32 between reclosers R1 and R2. Recloser R3 also acts exactly the same as it did in the previous example. It reconfigures to protect the section of line between reclosers R2 and R3, and closes, which brings the system 31 to the state shown in FIG. 10.
The permanent short circuit now causes a large current to flow from the substation, through reclosers R5, R4, R3, and R2, causing a temporary power degradation to Load 2, Load 3, Load 4, Load 5, and Load 6. When recloser R2 senses a large current after reconfiguring to protect the line between reclosers R2 and R1, it opens very rapidly, and remains open. It does not attempt to reclose. Because recloser R2 opens very rapidly, reclosers R3, R4 and R5 and circuit breaker 25 all remain closed.
The system 31 now resides in the state shown in FIG. 11. Notice that the permanent short circuit 28 between reclosers R1 and R2 only caused an extended power outage for the Load 2 connected between reclosers R1 and R2. Load 1 experienced one to several temporary power degradations as recloser R1 was testing the line for a temporary short circuit. Load 3 experienced one to several temporary power outages lasting several seconds as recloser R1 was testing for a temporary short circuit, and then experienced a longer power outage as reclosers R2 and R3 reconfigured. Finally Load 3 experienced a temporary power degradation when recloser R3 closed, causing short circuit current to flow through reclosers R2, R3, R4 and R5. Load 4, Load 5 and Load 6 all experienced a temporary power degradation lasting less than one second when recloser R3 closed. If the substation breakers 24 and 25 are located in the same substation 22, or in substations electrically close to each other, then Load 4, Load 5 and Load 6 may also experience temporary degradations in power due to the initial short circuit, and when recloser R1 tests the line for a temporary short circuit. A timeline for the entire process is shown in the bottom portion of FIG. 11.
In our final example of this configuration, assume a permanent fault 28 between reclosers R2 and R3, as shown in FIG. 12. This short circuit causes large short circuit current to flow through the substation breaker 24 and reclosers R1 and R2. Recloser R2 is programmed to open with less delay than the substation breaker 24 and recloser R1, so recloser R2 opens. The short circuit current ceases, so the substation breaker 24 and recloser R1 remain closed. Recloser R2 tests the line 32 several times for a temporary short circuit, and finally opens permanently. Each time recloser R2 opens, it does so before recloser R1 and the substation breaker 24 react, so each time recloser R1 and the substation breaker 24 remain closed.
The system 31 is now in the state shown in FIG. 13. As with the previous examples, recloser R3 is programmed to close after it senses either line connected to it is de-energized. When recloser R3 closes, a large short circuit current flows through reclosers R3, R4 and R5 and circuit breaker 25. Recloser R3 is programmed to open and not reclose in response to short circuit current. After recloser R3 opens, the system reverts to the state shown in FIG. 13. Notice that Load 1 and Load 2 only experienced one to several temporary power degradations while recloser R2 tested the line for a permanent fault. Load 4, Load 5 and Load 6 experienced one temporary power degradation when recloser R3 closed. Load 3 is the only load that experiences an extended power outage. A timeline for the entire process is shown in the bottom portion of FIG. 13.
All three of the previous examples could have included short circuits on the lower half of the distribution loop, and the results would have been similar except that breaker 25, reclosers R3, R4, and R5, and loads Load 4, Load 5, and Load 6 would have been involved.
Each of the reclosers in looped distribution lines has a set of rules for operation. From the preceding discussion, it might seem that each recloser performs different functions depending on the location of the short circuit. However, each recloser follows a certain preprogrammed sequence of actions regardless of the location of the short circuit. FIG. 14 shows a flow chart of the preprogrammed sequence of actions that are taken by recloser R3, FIG. 15 shows a flow chart of the preprogrammed sequence of actions that are taken by reclosers R2 and R4 and FIG. 16 shows a flow chart of the preprogrammed sequence of actions that are taken by reclosers R1 and R5.
Each flowchart, FIGS. 14 through 19, has a start and end bubble. The methods move from the end bubble or from anywhere in the flowchart to the start bubble when the scheme is reset or restarted. The reset or restart occurs after the permanent short circuit is repaired or when an operator determines the method should be reset. The reset or restart can be manual, such as when a person issues a reset signal or command to the recloser, or automatic, such as when the reclosers reset themselves when they detect some sufficient condition.
The above-described operation of recloser R3 is summarized in FIG. 14. After starting at bubble 40, recloser R3 determines if line 32 is de-energized for a predetermined time on one side only at decision block 41. If the line is energized on both sides of recloser R3, or de-energized on both sides of recloser R3, it continues to measure voltage on both side of recloser R3 until line 32 is de-energized on one side only for some predetermined time. If the line is de-energized on one side only for a predetermined time, the process proceeds to decision block 42 to determine if line 32 is de-energized to the right of recloser R3. If so, recloser R3 configures to protect line 32 to the right of recloser R3 at block 43. If not, recloser R3 configures to protect line 32 to the left of recloser R3 at block 44. In either situation, recloser R3 closes at block 45. It then continues to sense for short circuit current at decision block 46. If recloser R3 detects a short circuit current lasting longer than a predetermined time, recloser R3 opens at block 47 and the process terminates at end bubble 48.
The above-described operation of reclosers R2 and R4 is summarized in FIG. 15. After starting at bubble 50, reclosers R2 and R4 determine if a short circuit current is present on line 32 at decision block 51. If a short circuit is present, reclosers R2 and R4 determine at decision block 52 if the short circuit current lasts longer than a first predetermined time. If not, the process begins again at start bubble 50. If the short circuit current lasts longer than a predetermined time, then recloser R2 or R4 opens at block 58. Decision block 59 determines if the line has been tested for temporary short circuits more than a predetermined number of times. If it has, then the process ends at block 57. If the line has been tested for a temporary short circuit less than a predetermined number of times, then the recloser R2 or R4 is closed at block 60 and the process continues from the start bubble. If no short circuit current is detected at decision block 51, then a check is performed to determine if the line is energized at decision block 53. If the line is not de-energized, i.e., if the line is still energized, then the process continues from the start bubble. If the line is de-energized, then recloser R2 or R4 is reconfigured to protect the upstream line, i.e., recloser R2 or R4 is reconfigured to protect the line between reclosers R1 and R2 or between reclosers R5 and R4. Reclosers R2 and R4 again monitor line 32 for a short circuit current that lasts longer than a predetermined amount of time at block 55. If a short circuit current is detected and lasts longer than the predetermined amount of time, recloser R2 and/or recloser R4 open at block 56 and end the process at bubble 57.
The above-described operation of reclosers R1 and R5 is summarized in FIG. 16. After starting at bubble 61, reclosers R1 and R5 determine if a short circuit current is present on line 32 at decision block 62. If a short circuit is present, reclosers R1 and R5 determine at decision block 63 if the short current lasts longer than a first predetermined time. If the short circuit current does not last longer than a predetermined time, then the process reverts to the start bubble 61. If the short circuit lasts longer than a predetermined time, then recloser R1 or R5 opens at block 67. Decision block 68 determines if the line has been tested for temporary short circuits more than a predetermined number of times. If it has, then the process ends at bubble 66. If the line has been tested for a temporary short circuit less than a predetermined number of times, then recloser R1 or R5 is closed at block 69 and the process continues from the start bubble. If short circuit current is not detected at decision block 62, then recloser R1 or R5 determine if the line has been de-energized for longer than a predetermined time at decision block 64. If the line has not been de-energized for longer than a predetermined time, then the process reverts to start bubble 61. If the line has been de-energized for a predetermined time, then recloser R1 or R5 opens at block 65, and the process ends at bubble 66.
These processes in FIGS. 14-16 are sufficient to reduce the extended power outage to only the section of looped distribution line 32 containing the permanent short circuit 28. Notice that these processes cover only the function of the reclosers related to controlling the looped distribution line 32. Each recloser may perform various other functions, such as metering, reporting, etc. These other functions are not shown in FIGS. 14, 15 and 16.
A number of problems exist with respect to the present method of controlling looped distribution lines, as represented by FIGS. 14-16. The method implemented by the processes discussed above has several non-idealities:                A. Permanent short circuits 28 between reclosers R1 and R2, or between reclosers R2 and R3 cause a temporary power degradation to Load 4, Load 5 and Load 6 when recloser R3 closes. If the system were not configured as a loop, i.e. if recloser R3 did not connect the top distribution line to the bottom distribution line, then this temporary power degradation would not occur (assuming the looped distribution line terminated in electrically separated substations), or this temporary power degradation would be less severe. In other words, this arrangement decreases the quality of power supplied to the distribution line that does not have the short circuit 28 while it increases the quality of power supplied to the distribution line that does have a short circuit.        B. Permanent short circuits 28 between reclosers R1 and R2, or between reclosers R2 and R3, cause added stress on the power system when recloser R3 closes. This stress includes large short circuit currents that stress transformers, generators, conductors, etc. The added stress also includes decreased voltages that stress many types of connected electrical loads such as motors and electronics. The added stress also includes added wear on recloser R3 when recloser R3 must open after closing with a permanent fault between reclosers R2 and R3, and added wear on recloser R2 when R2 must open after recloser R3 closes with a permanent fault between reclosers R1 and R2.        C. If the power source in the substation is de-energized, then the distribution line is de-energized. Recloser R1 responds to this by opening (Blocks 64 and 65 in FIG. 16). This is a nuisance because after power is restored to the substation, the loads downstream of recloser R1 remain de-energized until recloser R1 is closed, possibly by a manual operation after several hours.        
The shortcomings described are also applicable to the other side of the loop in FIGS. 14-16, i.e., the side of the loop containing breaker 25 and reclosers R4 and R5.
There has been a long-felt need for methods or systems that efficiently and effectively reconfigure an electrical power distribution system to provide power to most of the loads upon the occurrence of a short circuit on the distribution line.
Accordingly, it is a general object of the present invention to provide improved methods and systems that reconfigure a looped distribution line in a manner that continues to supply power to most of the loads when a short circuit occurs.
Another general object of the present invention is to provide improved methods and systems for sectionalizing a looped distribution line to reduce the stress on the power system and on the power system components when a short circuit occurs.
Another general object of the present invention is to provide improved methods and systems for sectionalizing a looped distribution line to reduce the unnecessary outages in the distribution power system when no short circuit exists in the distribution power system.
Yet another object of the present invention is to provide a plurality of preprogrammed switches, with at least some of the preprogrammed switches having at least one unique open interval when responding to a short circuit, such that other preprogrammed switches can determine which preprogrammed switch opened in response to the short circuit.
A further object of the present invention is to provide a power distribution system and methods in which a normally open preprogrammed switch does not close until an adjacent preprogrammed switch opens when the short circuit is downstream from the adjacent preprogrammed switch.
A still further object of the present invention is to provide methods for determining which preprogrammed switch opened in response to the occurrence of a short circuit.